No reason at all. It's your game, you play it how you wish.
You'd also have considerably greater spending power with which to replace those players.
Not for me ... and I don't play with anyone else either. But that's one of the game's best features - it allows you to play your game, your way.
Whatever method you chose to adopt, incorporating relegation would require a major undertaking on the development team's part so I think we can all agree it's unlikely to happen. Interesting to hear the various views, nonetheless.